1. Field of the Invention
The invention relates to a control apparatus for an internal combustion engine and a control method thereof, and more particularly, to a control apparatus for an internal combustion engine that includes a plurality of cylinders and a changing mechanism that can change the timing for closing intake valves provided for each of the cylinders, and a control method thereof.
2. Description of the Related Art
In an engine that includes a plurality of cylinders, it is known that the air-fuel ratio may differ among the cylinders due to the variation of operational characteristics (valve duration, phase, etc) of the intake valves among the cylinders, and so on. When the air-fuel ratio differs among the cylinders, the fluctuation of output torque becomes large because the burning conditions of air-fuel mixture consisting of air and fuel differ. In this case, the fluctuation of rotation of the crankshaft (fluctuation in the rotation speed while the crankshaft makes one turn) becomes large, so the vibration and noise of the engine become large. Therefore, it is desirable to reduce such variation of air-fuel ratio.
Japanese Patent Application Publication No. JP-A-2002-303187 discloses a control apparatus for a multi-cylinder internal combustion engine (a multi-cylinder engine) that suppresses the variation of air-fuel ratio and torque among the cylinders. This control apparatus calculates the air-fuel ratio of exhaust gas (exhaust gas air-fuel ratio) of the cylinder for which the valve duration of intake valves is set to maximum, and based on the calculated air-fuel ratio, reduces the variation of fuel injection amount among the cylinders.
According to the control apparatus for the multi-cylinder internal combustion engine disclosed in the publication, the reduction of the variation of fuel injection amount among the cylinders is performed based on the air-fuel ratio of the exhaust gas which is calculated when the amount of intake air is not controlled using the valve duration of the intake valves, i.e., when the same amount of intake air is sucked into each cylinder. That is, the amount of fuel to be injected into each cylinder is corrected such that all the cylinders have the same exhaust gas air-fuel ratio. This makes it possible to suppress the variation of fuel injection amount among the cylinders. Thus, the variation of air-fuel ratio and torque among the cylinders can be suppressed.
However, as in the case of the control apparatus for the multi-cylinder internal combustion engine disclosed in Japanese Patent Application Publication No. JP-A-2002-303187, if the air-fuel ratio is calculated when the maximum valve duration of the intake valves is set, such valve duration may affect the air-fuel ratio. For example, when the intake valves of a certain cylinder close later because of their characteristic that the actual valve duration tends to be larger than set in the control, a larger amount of air is pushed back to the intake ports as the piston goes up. As a result, the amount of intake air in the cylinder decreases, making the air-fuel ratio lower. Meanwhile, when the intake valves of other cylinder close earlier because of their characteristic that the actual valve duration tends to be smaller than set in the control are provided, a smaller amount of air is pushed back to the intake ports as the piston goes up, and as a result, the amount of intake air in the cylinder increases, making the air-fuel ratio higher. In order to calculate the air-fuel ratio without being affected by the variation of operational characteristics (e.g., valve duration) of the intake valves for each of the cylinders, further improvement is still needed.
It is an object of the invention to provide a control apparatus for an internal combustion engine, which can obtain an air-fuel ratio that is not affected by the operational characteristics of the intake valves or a value corresponding to such an air-fuel ratio.